In a receiving device using a CDMA (code division multiple access) communication technique, a signal power of a received signal and an interference noise power for each finger are estimated, and the signal-power-to-interference-noise-power ratio (SINR) estimated thereby is used as a weighting factor for the maximum ratio combining of a demodulated signal The decrease of an SINR estimation accuracy deteriorates the accuracy of the maximum ratio combining and causes receiving characteristics to be deteriorated. Accordingly, it is necessary to improve the estimation accuracies of the signal power and the interference noise power.
The signal power and the interference noise power are normally estimated at a constant period, for example, at each increment of slot. Since a limited measurement interval, such as determined by a pilot signal, is generally used, the estimation accuracies of the signal power and the interference noise power in one slot are low. As a method for improving the estimation accuracies of the signal power and the interference noise power, there is a technique for averaging the estimated values each obtained in one slot among a plurality of slots.
However, as described in “a method for estimating high accuracy channel using a plurality of pilot blocks in DS-CDMA” written by Ando and Sawahashi (The Institute of Electronics, Information and Communication Engineers Technical Report, RCS96-72, 1996) (hereinafter referred to as a “document 1”), the improvement in the estimation accuracy by inter-slot averaging in the signal power is limited. The is because the fluctuation of the signal power with respect to time due to Rayleigh fading is significantly large to thereby lower the fading tracking capability, although the interference noise components included in the received signal are suppressed by the inter-slot averaging.
On the other hand, since the interference noise power is substantially constant with respect to time, an effect of improving the estimation accuracy of the interference noise power by the inter-slot averaging processing is large. The present invention is aimed at raising the SINR estimation accuracy by improving the estimation accuracy of the interference noise power and at preventing the characteristic deterioration of the CDMA receiving device.
FIG. 8 shows an example of the receiving device configured to averaging and estimating the interference noise power among a plurality of slots, such as described in “Examination of Method for Measuring SIR under Adaptive Transmission Power Control of DS-CDMA” written by Seio, Okumura and Doi (The Institute of Electronics, Information and Communication Engineers Society meeting B-330) (hereinafter referred to as a “document 2”). In FIG. 8, interference noise power estimated of one slot before is stored in a one-slot-before interference-noise-power memory 809. Interference noise power 807-n calculated in the current slot and interference noise power 810-n estimated one slot before are averaged by an interference-noise-power inter-slot averaging circuit 808-n. 
However, if the interference noise power becomes valid, immediately after the receiving device establishes a radio link to a transmitting device, or after the finger once becomes an invalid state, the estimated value for the one-slot-before interference noise power does not exist. It is to be noted that the recitation “finger is valid” means the state that synchronization timing is detected for the finger. Similarly, the recitation “finger is invalid” means the state that the synchronization timing is not detected for the finger.
In FIG. 8, if there is no estimated value for the interference noise power of one slot before, the one-slot-before interference-noise-power memory 809 does not store therein the interference noise power. At this time, the interference-noise-power inter-slot averaging circuit 808-n performs averaging processing between “0” and the current in-slot interference noise power 807-n. Therefore, if there is no estimated value for the interference noise power of one slot before, the interference-noise-power inter-slot averaging circuit 808-n outputs an interference noise power 811-n having a low estimation accuracy to an SINR calculation circuit 814. As a result, the estimation accuracy of the SINR is lowered, and the accuracy for the maximum ratio combining of the demodulated signals is deteriorated, thereby causing the characteristics of the CDMA receiving device to be deteriorated.